1. Field of the Invention
The present invention relates to an optical fiber splicing method, and more particularly, to an optical fiber splicing method for splicing opposing end faces of two optical fibers which are different in mode field diameter (MFD) from each other or two optical fibers which have small MFDs with a small loss.
2. Prior Art
In recent years, the transmission capacity has been dramatically increased thanks to the development of wavelength division multiplexing (WDM) transmission scheme in optical communication systems. Optical fiber lines routed in such a system which has a large transmission capacity are required to provide performances such as a reduced non-linear optical effect, a reduced wavelength dispersion, a smaller wavelength dispersion slope, and the like.
To meet the requirements, the following dispersion management line is now under investigation. The dispersion management line has, for example, a simple mode fiber (SMF, for example, 1300 nm zero dispersion optical fiber), and a dispersion compensating optical fiber for compensating the dispersion and the dispersion slope of the SMF (for example, dispersion compensating fiber: DCF, dispersion slope compensating fiber: SDCF, reverse dispersion fiber: RDF, and the like), which are spliced end-to-end by fusion, and is intended for use in high speed communications using, for example, light in a 1550 nm band.
The 1300 nm zero dispersion optical fiber, which is the simple mode optical fiber illustrated above, has a core made of silica doped with GeO2 and a clad made of pure silica, and exhibits MFD in a range of 9 to 11 μm at wavelength of 1550 nm. An MFD enhanced simple mode optical fiber in turn exhibits MFD equal to or more than 11 μm.
On the other hand, with a dispersion compensating optical fiber having a negative high dispersion characteristic, the relative index difference must be as high as approximately 3%. For this reason, the core is formed of silica heavily doped, for example, with GeO2, and the clad is formed of silica doped with fluorine. The core has a diameter of approximately 2 to 3 μm, which is extremely small as compared with the core diameter of a simple mode fiber. The MFD at wavelength of 1550 nm is approximately 5 μm. In other words, the dispersion compensating fiber is smaller in both core diameter and MFD than the simple mode fiber at wavelength of 1550 nm.
Therefore, simple fusion splicing of end faces of the two optical fibers would give rise to an optical loss due to light leak based on the difference in MFD at the splice, even if the optical axes of both fibers are in alignment with each other. For example, when a simple mode optical fiber having MFD of 10 μm is spliced to a dispersion compensating optical fiber having MFD of 5 μm by fusion with their optical axes placed in alignment, an optical loss at the fusion splice is on the order of 1.94 dB.
Generally, for addressing the generation of the optical loss in the fusion splice, a TEC method (Thermally Defused Expanded Core method) is applied to reduce the optical loss.
The TEC method applies a heating treatment to the fusion splice to diffuse dopants in the core into the clad to substantially increase the diameter of the core and MFD.
For example, when the TEC method is applied to a fusion splice of a simple mode optical fiber and a dispersion compensating optical fiber, the softening temperature of the clad (doped with fluorine) in the dispersion compensating fiber lower than the softening temperature of the clad (pure silica) in the simple mode fiber causes the dopant (GeO2) in the cores of both optical fibers to diffuse into the associated clads faster in the dispersion compensating fiber than in the simple mode fiber. Therefore, in the process of heating treatment, the dopant in the core of the dispersion compensating optical fiber diffuses more to advance substantial enlargement of the core diameter in the fusion splice, causing the core diameter of the dispersion compensating fiber to match with the core diameter of the simple mode optical fiber. In other words, the MFDs match to reduce an optical loss between both optical fibers.
The optical loss is reduced in the fusion splice in this way.
The splicing of optical fibers involves not only splicing of different types of optical fibers which are different in core diameter and MFD from each other, but also splicing of the same type of optical fibers for adjusting the length of an overall optical line and adjusting the characteristics.
For example, splicing may be performed with dispersion compensating optical fibers of the same type having an extremely small MFD and accordingly an extremely small core diameter. In this event, opposing end faces of the two optical fibers are likewise spliced by fusion using, for example, a fusion splicer.
In this case, however, the extremely small core diameter gives rise to a problem that even a slight misalignment of the cores would cause a large optical loss at the fusion splice. In addition, if a discharge fusion splicer, for example, is used for the fusion splicing, a sufficient reduction in optical loss has not been provided during splicing of recent fine cores even if a discharge condition is optimized for the discharge fusion splicer.
For this reason, the TEC method is applied to the formed fusion splice, even in such a splice, to diffuse dopants in the cores to enlarge MFDs in the fusion splice to prevent the optical loss due to the misalignment.
When the aforementioned dispersion management line is used, for example, in an optical submarine cable or the like, the fusion splice is required to have a high strength as well as provide a small loss.
In regard to an increase in the strength of the fusion splice, the following action has been conventionally taken. Specifically, before fusion splicing, the surface of an optical fiber is covered with a resin protection layer for reducing or removing the influence of factors which deteriorate the strength of the optical fiber, such as a contact of the optical fiber with a fiber cutter, a V-groove of a fusion splicer in which the optical fiber is placed, a fiber clamp for fixing the optical fiber, and the like.
However, the formation of the resin protection layer causes not a few tackiness to remain on the surface. This may result in a loss of progressivity of the optical fiber which would serpentine, or prevent the two optical fibers from progressing at a timing at which they should progress at the fusion splice. As a result, the amount of shift of the core is larger as compared with an optical fiber formed with no resin protection layer, leading to an increased optical loss in the fusion splice.
Particularly, when an optical fiber having a small MFD is formed with the resin protection layer for fusion splicing with a high strength, even a slight misalignment of the core should be avoided for reducing an optical loss. Actually, however, this is a quite difficult operation.
As appreciated from the foregoing, though the formation of the resin protection layer on the surface of an optical fiber to be spliced may be effective means for increasing the strength of a fusion splice, on the other hand, this introduces a further increase in optical loss in the fusion splice as compared with an optical fiber formed with no resin protection layer.
The heating treatment performed after the fusion splicing generally involves discharge, hydrogen/oxygen burner flame, and propane/oxygen burner flame.
However, the discharge heating merely locally heats the fusion splice at high heating temperatures. The fusion splice is treated in a relatively short time and suddenly cooled down. This gives rise to a problem that the dopant in the core is likely to diffuse with instability, and distortion is accumulated in glass of the fusion splice.
Moreover, the discharge heating encounters difficulties in optimizing the discharge condition, properly controlling a heating temperature, and properly locating a site to be heated. For this reason, when the discharge heating is repeated a plurality of times, the fusion splice suffers from a varying outer diameter (so-called constriction), resulting in a reduced diameter and simultaneous degradation of strength.
On the other hand, with burner flame based heating, a proper temperature control is facilitated as compared with the discharge heating, and a site to be heated is appropriately narrowed down with ease. On the other hand, however, an optical fiber remains recumbent, and a burner flame is applied to a softened fusion splice, so that the pressure of the flame and the self weight of the optical fiber cause a bending deformation in the fusion splice, possibly increasing an optical loss.
Such a problem is addressed by heating the fusion splice while it is applied with a tension in the axial direction. However, depending on the magnitude of a tension to be applied, the softened fusion splice will draw, and as is the case with the repeated discharge heating, the fusion splice will be formed with a reck to introduce a reduced strength of the fusion splice and large variations in strength.
An optical fiber in such a state can give rise to a break when the fusion splice is bent, due to a stress concentrating thereon.